(1) Field of the Invention
The present invention relates to the field of display devices. More specifically, the present invention relates to the field of flat panel display devices having adjustable gamma responses.
(2) Background
Flat panel or liquid crystal displays (LCDs) are popular display devices and are used in computer systems, multi-media systems and other consumer electronic devices. Many types of flat panel displays are typically back-lit or edge-lit. That is, a source of illumination is placed behind the LCD layers to facilitate visualization of the resultant image. The LCD material acts as a regulator of light passing through color filters and to the eye, thereby forming an image. Flat panel LCD units are used today in many applications including the computer component industry and the computer peripheral industry where flat panel LCD units are an excellent display choice for lap-top computers and other portable electronic devices. Flat panel LCD displays are also being used in the high end graphic arts industry where display quality and realistic image quality are very important consumer goals.
In the field of flat panel LCD devices, much like conventional cathode ray tube (CRT) displays, a pixel is composed of a red, a green and a blue color point or xe2x80x9cspot.xe2x80x9d When each color point of the pixel is illuminated simultaneously, and with the appropriate intensity, white can be perceived by the viewer at the pixel""s screen position. To produce different colors at the pixel, the intensities (e.g., brightness) to which the red, green and blue points are driven are altered in well known fashions. The separate red, green and blue data that correspond to the color intensities of a particular pixel are called the pixel""s color data. Color data is often called gray scale data. The degree to which different colors can be achieved by a pixel is referred to as gray scale resolution. Gray scale resolution is directly related to the amount of different intensities (e.g., luminosities) to which each red, green and blue point can be driven.
All color systems must accurately reproduce the color tones of the original scene. Since no known reproduction process can exactly capture the original elements in a given situation (e.g., the brightness of the sun shining down on a landscape), the basic goal of reproduction is to capture the relative differences between objects in the original view. The ratio of the whitest point to the blackest point in a scene is know as its dynamic range, which must be reproduced on some medium such as film, a CRT, an LCD, or paper. The characteristics of this medium, or its xe2x80x9cnative response,xe2x80x9d will determine the level of success a given reproduction achieves.
The number of steps, or gray scale, into which this dynamic range can be subdivided determines the resolution of a particular primary color. A typical monitor system will have the ability to display 8-bits, or 256 shades per primary color for a total of over 16.7 million colors (256xc3x97256xc3x97256). This is known as the color depth or image palette of the display system.
These display mediums, especially CRTs, although they behave in a fairly linear fashion, introduce some amount of distortion which has to be corrected to make the reproduced image look xe2x80x9cproper.xe2x80x9d Human eyes see logarithmically. To compensate for this, monitors are made to mimic the eye""s viewing so that the display shows the eye information in a way people are used to seeing. The resulting response curve varies in an exponential manner known as the xe2x80x9cgamma curve,xe2x80x9d which is a polynomial equation describing any point on a brightness curve being displayed by a particular monitor. Its function is to correct for the non-linearity of the input signal and its corresponding luminance. Using CRTs as an example, the brightness changes very little at the lower energy gray levels causing some compression of the shadow detail where our eyes are the most sensitive. So instead of a straight-line, linear response where there is a equal amount of output change for a given input change, the gamma curve has a long, shallow beginning before it begins to climb.
Traditionally, display management systems manipulate the shape of the gamma curve through the use of color lookup tables (xe2x80x9cLUTsxe2x80x9d) in a memory location some where in the data information path on the host computer or graphics card. The LUT contains one entry, typically, per gray scale level. The information bits that make up an image are converted from one value to another, as in the following equation, on their way to the display device. This method has farther reaching applications.
RGBc=RGBcxe2x88x92max[[xe2x88x92kop+(RGBin/(RGBinxe2x88x92max))(1/xc3x97)]/Kgp]
where:
RGBc=corrected RGB value
RGBcxe2x88x92max=maximum of corrected RGB values (2nxe2x88x921)
RGBin=RGB input value
RGBinxe2x88x92max=maximum of RGB input values (2nxe2x88x921)
Kop, Kpg, x=fitted parameters of display primary channel
By using LUTs to alter the gamma curve, the host computer system or graphics card can impact the luminance (brightness) as well as the color temperature of the monitor.
There are strong historical reasons why particular gamma responses have been selected by different display customer segments. For instance, different displays may have different gamma responses. In some instances, however, the gamma response of a display is based on the type of work for which display is used. For instance, a digital television (TV) or web based TV may use a different gamma response than a high end graphics computer aided design system, etc. Further, print media may select 1.8 gamma and film media may use 2.2 gamma. Further, the gamma response of a display device may be altered depending on the environment in which the screen is used, e.g., the lighting characteristics of the office, etc. The displays available today solve these issues on an independent, case-by-case basis. Different displays may have different gamma responses. Therefore, it is advantageous to provide a display device that has an adjustable gamma response. It is further advantageous to provide a display device that could give the display added flexibility to solve all such applications well.
There is a disadvantage in using LUTs to correct the normal gamma setting, the color temperature, and the brightness at which a particular display operates: the overall luminance and color resolution (e.g., the xe2x80x9ccolor depthxe2x80x9d) of the displayed data can be compromised by using LUTs to correct the normal gamma setting. Adjusting these three image elements in this manner can use up to 2 bits of information in a memory register that would normally be applied toward describing the tonal qualities of the image itself. Loss of 2 bits of color depth in an 8-bit system means that the user is being deprived of 16.52 million colors (28xc3x9728xc3x9728xe2x88x9226xc3x9726xc3x9726) out of a possible palette of 16.78 million. Traditional solutions increase the addressable memory depth in the host or the graphics card to 10-bits in order to over-sample the RGB input, apply the correction, and then down-sample to preserve the original column information. However, this is costly and difficult to implement. Therefore, using LUTs to correct the normal gamma setting can lead to a degradation of the overall gray scale resolution of a monitor. Therefore, it is advantageous to provide a display device that has an adjustable gamma response without an incidental degradation of gray scale resolution.
Accordingly, it would be advantageous to provide a display that allows its gamma response to be altered without any incidental loss in gray scale resolution. It would also be advantageous to provide a display that allows its gamma response to be altered, via software control, without any incidental loss in gray scale resolution. It would also be advantageous to provide a flat panel LCD display that allows its gamma response to be altered, via software control, without any incidental loss in gray scale resolution. It would be advantageous to provide a display that can alter its gamma without requiring external RGB gamma LUTs in the host computer. These and other advantages of the present invention not specifically described above will become clear within discussions of the present invention herein.
Embodiments of the present invention offer a novel solution to this problem in that the gamma of the display system can be altered within the flat panel itself rather than resorting to LUTs in graphics memory locations. Coupled with inventions disclosed in previous filings pertaining to separate backlight color adjustment methods and separate backlight luminance control such as Ser. No. 09/087,280 entitled xe2x80x9cA LIQUID CRYSTAL FLAT PANEL DISPLAY WITH ENHANCED BACKLIGHT BRIGHTNESS AND SPECIALLY SELECTED LIGHT SOURCESxe2x80x9d filed on Jun. 26, 1998, Ser. No. 09/087,745 entitled xe2x80x9cA MULTIPLE LIGHT SOURCE COLOR BALANCING SYSTEM WITHIN A LIQUID CRYSTAL FLAT PANEL DISPLAYxe2x80x9d filed Jun. 26, 1998 and Ser. No. 09/187,161, entitled xe2x80x9cA method and System for Providing a Colorimetric Reference Profile for a Flat Panel Monitorxe2x80x9d filed Nov. 11, 1998, each of the three main elements of the display system may be controlled separately and independently of any other element. The present invention is especially useful for graphic arts, multimedia, and film production applications where preserving color depth and maintaining color control is critical.
Future flat panel displays may incorporate more than three color primaries, thereby offering the user a greater color space. This invention applies even to that wider gamut in retaining more color information by maintaining independence between the critical elements of the display and the print or film output segments of color work product.
A flat panel display is described herein having a programmable gamma without incidental loss in gray scale resolution. In one embodiment, the flat panel display is a liquid crystal display (LCD). The invention includes a method of applying and adjusting a set of gamma controlling voltages to the DC reference circuit (a.k.a. ladder voltages) of an LCD module producing a change in the gamma response of the LCD module without incidental loss of gray scale resolution. An Adjustable Ladder Circuit (ALC) is thereby realized. Separate ALCs can be provided for red, green and blue primaries. By adjusting, in a predetermined fashion, the reference voltages applied to the row and column drivers of an LCD display, the gamma response of the LCD can be changed to a different value. Because the input digital signals are not affected, the same color resolution and dynamic range are maintained, but the gamma now is a function of the setting of the reference voltages and is not a function of the contents of any gamma LUTs (located within the host) which are not required by the present invention.
The DC reference circuit, in its simplified form, is a multi-node voltage divider circuit. These reference voltage nodes are applied to the row and column drivers of the LCD module to control the ON/OFF states of each red, green and blue sub-pixel. The reference voltages corresponding to these nodes can be altered in accordance with the present invention to achieve a desired gamma response. The input digital signals provided by the host""s graphics source modulate these voltage nodes to produce the desired gray scale value, applying across the LCD sub-pixel a percentage of DC reference voltage.
Specifically, one embodiment of the present invention includes a monitor comprising: a flat panel display screen comprising rows and columns; a circuit producing the LCD""s DC reference voltages which are supplied to row and column driver circuits; a timing controller producing the timing signals applied to the row and column driver circuits; and a controller device for applying and adjusting gamma controlling voltages to the DC reference voltages. These controlling voltages alter the gamma response of the flat panel display screen without degrading its gray scale resolution.
Further embodiments include a monitor wherein the controller device is a source of programmable voltages and wherein the source of programmable voltages comprises a plurality of summing amplifiers for each node of the DC reference circuit. Further embodiments include a monitor circuit wherein the flat panel display screen is a liquid crystal display device and wherein the flat panel display further comprises a plurality of row drivers and a plurality of column drivers to perform digital to analog conversion and wherein the controller device supplied the voltages to the plurality of row and column drivers.